1. Field of the Invention
The present invention relates to an active matrix substrate, and particularly to an active matrix substrate adapted for improving display quality of a liquid crystal display (LCD) using the same.
2. Description of Related Art
In order to satisfy the modern living, video and image devices are developing towards smaller sizes. Although conventional cathode ray tube (CRT) displays still have some advantages, with respect to the structures of their internal electron cavities, they are too bulky and occupy too much space. They even generate radiation which is harmful to eyes when displaying images. Therefore, flat panel displays such as LCDs, incorporating with new optoelectronic developments and semiconductor fabricating technologies, have gradually become mainstream display products.
FIG. 1A is a top view of a conventional thin film transistor (TFT) array substrate, and FIG. 1B is a schematic cross-sectional view along line a-b of the TFT array substrate shown in FIG. 1A. Referring to FIGS. 1A and 1B, the conventional TFT array substrate 100 includes a glass substrate 110, a plurality of scan lines 120, a plurality of data lines 130 and a plurality of sub-pixels 140. The scan lines 120, the data lines 130 and the sub-pixels 140 are all disposed on the substrate 110. The sub-pixels 140 are electrically connected with corresponding scan lines 120 and data lines 130. Each sub-pixel 140 includes a TFT 142 and a transmissive conductive electrode, such as indium tin oxide (ITO) electrode 144. The TFTs 142 are electrically connected with corresponding scan lines 120 and data lines 130, and the transmissive conductive electrode 144 is electrically connected with the TFT 142.
It is to be noted that, after the TFT array substrate 100 is assembled with a color filter substrate (not shown) and an LCD panel (not shown) is configured by filling a liquid crystal therein, any three adjacent sub-pixels 140 of each row of the TFT array substrate 100 respectively correspond in sequence to color filtering films for different colors on the color filter substrate. For example, the three adjacent sub-pixels 140 correspond in sequence to color filtering films for red color R, green color G, and blue color B. While the LCD panel is displaying, light passes through the liquid crystal layer first, and then passes through different color filtering films, respectively, so as to obtain red light, green light and blue light, all of which are combined to display different colors for viewers.
In a typical conventional LCD panel, the color filtering films (not shown) corresponding to the sub-pixels 140 are often distributed in strip arrangement. As shown in FIG. 1A, such arrangement often results in lower spatial resolution in horizontal direction. Sub-pixels 140 of a same column (in vertical direction) correspond to color filtering films of a same color, and sub-pixels 140 of a same row (in horizontal direction) correspond to color filtering films of three different colors, such as red, green and blue. Therefore, the sub-pixels 140 of a same row are periodically distributed with a period of the distance of three sub-pixels 140, thus more stripes may likely occur. Furthermore, as human eyes are less sensitive to blue color, thus a full white image may seem relatively dark, in that columns of blue sub-pixels 140 are arranged in series.
FIG. 2A is a top view of a conventional TFT array substrate, and FIG. 2B is a schematic cross-sectional view along line a-b of the TFT array substrate shown in FIG. 2A. Referring to FIGS. 2A and 2B, a better display quality can be obtained if sub-pixels 240 corresponding to color filtering films of respectively red color R, green color G and blue color B are distributed in delta arrangement as shown in FIG. 2A. However, such arrangement requires the corresponding TFTs 242 to be rearranged accordingly. Therefore, an original simple driving method, in which each data lines 130 controls sub-pixels 140 for displaying a single color as shown in FIG. 1A, has to be altered into a complicated one, in which each data line 230 controls sub-pixels 240 for displaying two colors as shown in FIG. 2A.
Moreover, parasitic capacitance Cpd caused between the transmissive conductive electrodes 244 and the data lines 230 also has to be considered. The parasitic capacitance Cpd increases as the transmissive conductive electrodes 244 are too close to the data lines 230, thus the displaying of the pixels will be interfered and cross-talk may occur when signals applied to the data lines 230 change. A dielectric layer (not shown) having a relative low dielectric constant employed between the data lines 230 and the transmissive conductive electrodes 244 may reduce the parasitic capacitance Cpd. Such a dielectric layer can be made of inorganic materials, organic materials or color filtering films, which can increase the aperture ratio by overlaying the transmissive conductive electrodes 244 onto the data lines 230. FIG. 2C schematically illustrates the capacitance effect of a single sub-pixel of FIG. 2A. Referring to FIG. 2C, a parasitic capacitance Cpd′ is generated between the transmissive conductive electrode 244 of a sub-pixel 240; and a data line 230 (the (n−1)th data line) disposed at its left side, and a parasitic capacitance Cpd is generated between the transmissive conductive electrode 244 of a sub-pixel 240 and a data line 230 (the nth data line) disposed at its right side. The total parasitic capacitance between the transmissive conductive electrode 244 and the data lines 230 is a sum of Cpd′ and Cpd. When an LCD panel including the TFT array substrate shown in FIG. 2A is driven by a dot inversion or a column inversion driving method, which means that the voltage differences of the nth data line and the common lines are positive (or negative), while the voltage differences of the (n−1)th data line and the common lines are negative (or positive) when a scan line be turned on. Therefore, the total parasitic capacitance (Cpd′+Cpd) can be reduced by cancellation of the parasitic capacitances Cpd′ and Cpd.
During fabrication of LCD panels with high aperture ratio, the transmissive conductive electrodes 244 are overlay on the data lines 230, the difference of the parasitic capacitances Cpd′ and Cpd are generally determined by the areas that the transmissive conductive electrodes 244 overlaying on the data lines 230. However, although the photo masks are preferably designed to have areas of the transmissive conductive electrodes 244 respectively the left side and the right side overlaying on the data lines 230 substantially equal to each other, in fact, practical exposing equipments are often hard to collimate sophistically enough to avoid an overlay shift between layers, especially when fabricating large size panels. Too much overlay shift between the transmissive conductive electrodes 244 and the data lines 230 causes too much difference between the absolute values of the parasitic capacitances Cpd′ and Cpd, and too much total parasitic capacitance, thus lowering the display quantity of the pixels.